Cu²⁺ Ions as a Paramagnetic Probe in EPR Studies of Radicals Generated Thermally in Starch

Abstract

Abstract Electron paramagnetic resonance (EPR) spectroscopy, with Cu 2+ ions as a paramagnetic probe, was applied to study the process of radical generation in starch and changes occurring in its structure upon thermal treatment and oxidation. Thermally generated radicals in starch containing Cu 2+ ions are localized on carbon atoms different from those in the pure starch. EPR signal parameters of the radical species ( g av = 2.0047 – 2.0049; A av = 0.9 – 1.0 mT) suggest a lower number of oxygen atoms in the vicinity of radical centres than that observed in pure starch and weakening of the interaction of the unpaired electron with the nuclear magnetic moment of the β hydrogen (for pure starch g av = 2.0059 – 2.0065; A av = 1.2 – 1.3 mT). A mechanism of thermal generation of radicals in the native and oxidized starch containing Cu 2+ ions was proposed. It consists of breaking of RCO‐Cu 2+ or RCOO‐Cu 2+ bonds in the native and oxidized starch, respectively, reduction of Cu 2+ to Cu + ions and formation of transient RCO• or RCOO• species, active in abstraction of hydrogen from the neighbouring C atom of the glucose unit, at which the unpaired electron became localized.